获取元素的精确坐标
var getCoords = function(el){var box = el.getBoundingClientRect(),doc = elem.ownerDocument,html = doc.documentElement;body = doc.body,win = getWindow(doc),clientTop = html.clientTop || body.clientTop || 0,clientLeft = html.clientLeft || body.clientLeft || 0,scrollTop = win.pageYOffset || html.scrollTop || body.scrollTop,scrollLeft = win.pageXOffset || html.scrollLeft || body.scrollLeft,top = box.top + scrollTop - clientTop,left = box.left + scrollLeft - clientLeft;return { top: top, left: left };}function getWindow( el ) {return isWindow( el ) ?el :el.nodeType === 9 ?el.defaultView || el.parentWindow :false;}function isWindow( obj ) {return obj && typeof obj === 'object' && 'setInterval' in obj;}